1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of one, or several, dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In lithographic apparatuses the size of features that can be imaged on to the wafer is limited by the wavelength of the projection radiation. As shorter wavelengths are used in order to be able to image smaller features, the dosage of the radiation increases, since the energy of the radiation is indirectly proportional to its wavelength. It has been found that conventional lithographic apparatuses suffer from the drawback that the dosage of the radiation causes the wafer to heat up on both a local and a global scale. Typically, the lithographic apparatus carries out a series of steps in order to form a sequence overlapping patterns on the wafer. One consequence of the wafer heating up is that overlay problems arise, that is errors are caused by overlay errors between the different layers of patterns on the wafer. It has further been found that for large radiation dosages, such as those typically encountered in lithographic apparatuses operating in the ultraviolet, deep ultraviolet, extreme ultraviolet and shorter wavelengths, wafer heating errors are becoming a contributing factor to the total overlay error. Further, conventional techniques for cooling wafers, for example, as described in U.S. Pat. No. 5,220,171, do not address the problem of local wafer heating.
In particular, it has further been found that due to the relative high thermal resistance of the wafer table and contact resistance of the wafer to wafer table and the wafer table to the exposure chuck, local conditioning remains a problem in conventional lithographic apparatuses. It has further been found that a major problem with respect to wafer heating in general, is to getting the heat out of the wafer fast enough. It has been found that good thermal conditioning of portions of the wafer table, for example, the mirror block, in conventional apparatuses only gives a small effect because of the poor thermal conduction of the wafer table material.